Hepatitis C virus (HCV) is a recently characterized RNA virus which is the major etiologic agent of non-A non-B (NANB) hepatitis. HCV results in substantial morbidity and mortality worldwide, with an estimated 50% of cases resulting in chronic hepatitis and 10 - 20% resulting in cirrhosis. Little is known presently about the host immune response to this virus. In a number of other viral infections, cytotoxic T lymphocytes (CTL) have been shown to be an important protective host defense. Our laboratory has extensive experience investigating antiviral CTL, characterizing the CTL response to HIV-1, establishing HIV-1 specific CTL clones in long-term culture, and mapping viral epitopes recognized by these CTL. The goal of this research effort is to use a similar approach to study the cell-mediated immune response to HCV, utilizing recombinant vaccinia viruses to express HCV antigens in target cells. Specifically, we propose to a) evaluate liver biopsy-derived lymphocytes and peripheral blood lymphocytes from HCV-seropositive subjects for the presense of HCV-specific CTL, b) determine the phenotype of cells mediating this response, as well as the role of HLA antigens in restricting HCV-specific cytolysis, c) determine the CTL epitopes recognized by HCV-specific CTL, and d) determine whether a correlation exists between HCV-specific CTL activity and the extent of liver damage. The information provided by these studies should enhance our understanding of the immunopathogenesis of HCV infection, and should be useful in the rational design of therapeutic interventions and vaccine strategies.